Expanded Tip Pile

ABSTRACT

Methods are provided for introducing unhardened highly expansive grout into the bottom of a formed shaft in the earth and subsequently forming the pile above the grout. The expansion of the hardening grout, after the curing of the pile, generates expansion stress on the surrounding soil and upwardly directed force to the pile. Methods of pile load capacity testing use expansive grout to generate a test load.

BACKGROUND

Concrete piles within the ground that are intended for supporting structures such as buildings are conventionally prestressed from below after their formation to increase and ensure their load-carrying capacity. The present invention pertains to methods and devices for applying mechanical stress to such structural piles located within the ground.

Conventional post-stressing of foundation elements includes methods of pumping fluid grout through a delivery system or device to the toe of a previously poured and cured pile. The externally generated fluid pressure induces stress both on the pile and on the soil at and below the pile toe. This stress acting in the downward direction compacts the soil below the tip of the pile, stiffening the end-bearing reaction, limiting settlement at design load.

In other conventional methods, hydraulic jacks are used to mechanically induce similar stresses on the foundation element and the supporting soil to produce similar effects. Like the prior methods using grout, the stresses are induced by pressure generating device at the ground surface level and require transporting a liquid to the pile tip. This process is complex and expensive and requires introducing liquid transport conduits into the pile structure.

SUMMARY OF THE INVENTION

The present invention includes methods of introducing unhardened highly expanding grout into the bottom of a formed shaft in the earth and subsequently forming a rigid pile above the grout. Expansion of the hardened grout, after substantial hardening of the pile, generates expansion stress on the surrounding soil and upwardly directed force to the pile to provide stability when bearing loads are applied to the pile. Unlike prior methods, no pressurized liquid is required to be pumped from the ground surface level to the pile tip.

In a preferred embodiment, expansive grout is introduced into multiple layers of glass fiber sheet material. An assembly of the combined grout and glass fiber material is placed at the bottom of a shaft before a pile is formed. The invention includes methods of forming piles and inducing stresses at the pile tip by locating at the pile tip highly expansive grout in combination with glass fiber sheet material.

Expanding grout may also be used according to the invention in novel methods of testing to determine load capacity characteristics of a pile. A volume of highly expanding grout is introduced into the bottom of a formed shaft in the earth and a concrete pile formed in conventional matter above the grout. Stress and displacement data associated with the expansion of the grout is obtained and pile load characteristics determined in conventional manners.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are cross section views of steps in the formation of a pile in a shaft according to one embodiment of the inventive method.

FIG. 2 is a perspective view of a novel grout mat according to the invention enabling the inventive methods.

FIG. 3 is one configuration of the inventive methods using the grout mat of FIG. 2 in a shaft in forming a pile.

FIG. 4 illustrates an inventive grout mat secured to a reinforcing cage.

DETAILS OF EMBODIMENTS OF THE INVENTION

FIGS. 1A-1C depict various steps in one embodiment of the inventive method in forming a prestress pile. In FIG. 1A, a quantity of liquid unhardened highly expansive grout 20 is deposited in the bottom of a shaft 100 formed in the earth in the conventional manner in preparation of forming a concrete pile. The term “shaft” herein means an open elongated vertical cavity configured for receiving a quality of unharded concrete for forming a pile within the shaft.

Herein, the term “highly expansive grout” (HEG) is used to define a grout or concrete-like material that has relatively high dimensional expansion upon curing and also a strength comparable to structural concretes. Typically, HEG materials may be formed of various different combination of portland cement, high alumina cement, CaSO₄.2H₂0 (gypsum); CaSO₄.½H20 (gypsum plaster); CaOH₂ (hydrated lime); CaO (quick lime); the prominant constituent being portland cement. Additives to alter characteristics may also be included. In all cases, for purposes here a HEG must produce substantial expansion after hardening sufficiently to reduced compressive stresses, such that resistance of expansion after hardening will result in substantial induces stresses.

The effective quantity and vertical depth of the HEG 20 may be dependent on the nature of the earth and the dimensions and desired load capacity of the pile to be formed. These requirements can be determined in conventional manner in any particular application.

In FIG. 1B, above the unhardened HEG 20, a quantity of conventional unhardened concrete 24 is introduced into the shaft 100 in conventional fashion for forming a pile. The concrete 24 is in direct communication with the unhardened HEG 20. Initially, both the concrete 24 and HEG 20 are unhardened.

The HEG 20 is selected or formulated to delay its expansion substantially until the concrete is hardened. Depending on the application, the characteristics of the HEG 20 may result that it has hardened and begun to expand as the concrete 24 hardens and before the concrete reaches full compressive strength.

What is critical is that the expansion of the HEG 20 be delayed such that the stresses induced by its expansion do not exceed the strength of the hardening concrete 24 at any point in time and that substantial load not be applied to the pile until it has substantially bonded to the surrounding earth. This may be influenced by the HEG 20 volume, the nature of the surrounding earth and the designed prestress load to be achieved.

When used with most conventional concretes, a delay in the HEG hardening of one to two days is desired. The expansion of the HEG can be delayed by retarding the hydration of reaction using chemical retarding agents. Similar agents may be found in conventional grouts used for demolition purposes, but these grout mixes are designed for a typical delay, from introducing of the grout to the shaft, of about four to eight hours and this is not effective for the purposes here. Proper formulation with known retarding agents can achieve delays in the range of the desired one to two days.

FIG. 1C depicts the concrete substantially hardened into a pile 26 such that it has substantial strength to compression stress and has established the desired adherence to the surrounding earth of the shaft 100. The HEG 20 has substantially hardened and continues to expand. The constrained yet expanding HEG 20 develops compressive stresses that generate an upward force on the pile 26. At the same time, these stresses compact and stress the earth surrounding the HEG 20.

In preferred embodiments, the introduction of the HEG into the shaft is assisted by a carrier material. FIG. 2 illustrates a configuration of a grout mat for introducing HEG into a shaft. FIG. 3 is an enlarged side view of the same mat. The grout mat 30 includes multiple layers 32 of flexible sheet fiberglass material, together with HEG. Each layer 32 preferably matches the size and geometry of the cross-section bottom of the shaft in which a pile is to be formed. For this reason, the mat geometry may be different for each application. The primary function of the fiberglass layers is to retain and carry the HEG to the bottom of the shaft and to hinder and limit mixing of the grout with the concrete in the shaft when the pile concrete is first introduced over the mat 30.

Each mat layer 32 may be formed of a sheet of woven fibers or bundles of fibers of fiberglass as is found in industrial fiberglass products. Conventional E-glass fibers may be used, or other forms such as R-glass or S-glass. While a woven construction is effective, other layer constructions, or combinations of constructions will likely provide similar results.

The HEG may be applied as a fluid to each layer 32 and the layers then assembled. Alternatively, the HEG may be combined as a powder with the layers 32 to form an entirely dry assembly, and water added at the time of application. On a volume basis, the mat 30 should contain about fifty percent (50%) HEG in the wet state and should be substantially without voids. FIGS. 2 and 3 are not to scale and the portions of fiberglass layers and the grout as shown are not representative of the relative quantities of each. The HEG should completely fill all voids in the construction of the fiberglass layers. Additional HEG will likely fill a volume between the layers and this is shown as a grout layer 21 between the fiberglass layers 32 in the figures.

It is critical that the mat 30 not be internally restrained in the axial direction (perpendicular to the plane of the layers). After placement in a shaft and during curing, the HEG expand and may separate adjacent layers. Expansion laterally (radially)—in the plane of the layers may be inhibited by the glass fibers. Any such action will be beneficial in increasing axial expansion or stress. In the cured state, the fiberglass layers 32 provides structural stability to the grout.

To achieve the needed expansion, the finished assembled mat 30 should have a thickness dimension TD in the range of ½ inch to six (0.5 to 6) inches, depending on the amount of expansion required and the nature of the grout used.

In use, the mat 30 may be deposited at the bottom of the shaft in any of a variety of ways. It may be convenient to secure the mat to incidental structures also being introduced to the shaft. FIG. 4 illustrates the inventive grout mat 30 secured to a reinforcing cage 40 for introduction into a pile shaft before forming a pile. The reinforcing cage 40 may be formed in the fashion of conventional reinforcing structures intended for strength reinforcing of concrete piles. Both the reinforcing cage 40 and mat 30 are depicted as having a rounded geometry, but it should be clear that any geometry may be used to accommodate the geometry of a particular shaft. Preferably, the mat 30 completely covers the bottom of the shaft.

The mat 30 may be secured to the reinforcing cage 40 in any of a variety of ways such as with fiber or metallic ties passing through a portion of the mat at its perimeter and encircling an element of the reinforcing cage 40.

Introduction of expansive grout by any of the methods discussed here may be used as part of a novel method of measuring the level of post-stress exerted on a pile by monitoring reaction of the pile. Monitoring may be carried out by any of a variety of conventional methods and devices. The results of this monitoring may be used to predict the future settlement behavior of the pile.

The following describes use of HEG in methods of load capacity testing of formed piles. Generally, a volume of highly expanding grout is introduced into the bottom of a formed shaft in the earth and a concrete pile formed in conventional matter above the grout. Stress and displacement data associated with the expansion of the grout is obtained and pile load characteristics determined in conventional manners.

In some prior known test methods, often referred to as “Osterberg” or bi-directional” tests, hydraulic jacks are used to apply a test load to the bottom of the formed pile. In the inventive methods HEG provided the function of the hydraulic jacks.

The test procedure initial steps are similar to the inventive post-stressing methods discussed above but with use of additional instrumentation to monitor the stress created by the grout and measure the pile deflections upward. Strain gages can be installed in the pile to further analyze soils resistance at diffract depths but are not necessary.

A pressure sensor is installed between the HEG and pile-forming concrete. The pressure sensor measures the stress created by the expansion of the HEG. The stress may be multiplied by the horizontal cross-sectional area of the HEG volume to get an estimate of the total load being created by the expansion event. As the base of the pile expands, the top of the pile may deflect upwards and instrumention is provided in conventional manner to detect and measure this deflection. This deflection, plotted with the calculated load, are the two variables of a load-deflection curve. Telltale rods can be anchored below the HEG to pass unrestrained to the top of the pile. The telltale rods are used to measure the change in overall length of the pile. This change is the total axial expansion of the grout and, when subtracted from the upward deflection of the top of pile, measures the downward deflection of the pile toe. The combined data from deflection, overall expansion and pressure measurement is used in known methods to determine a load capacity characteristic of the pile. The above described instrumentation is not illustrated in the figures but is well known in the industry.

The invention may be carried out by use of other mechanisms and devices while employing the novel aspects of the invention specified herein. The scope of the invention is defined by the following claims. 

1. Methods of forming a pile in the earth comprising: forming a vertical shaft in the earth; introducing a volume of highly expansive grout (HEG) into the shaft; introducing unhardened concrete into the shaft to form a pile above and connected to the volume of HEG; delaying the hardening of the HEG until the concrete is hardened; such that expansion of the HEG during hardening generates a uplifting stress on the pile.
 2. A method, according to claim 1, and wherein: the step of delaying the hardening of the HEG comprises delaying the hardening of the HEG by a period in the range of one to two days.
 3. A method, according to claim 1, and wherein: the step of delaying the hardening of the HEG comprises introducing a chemical agent to the unhardened HEG.
 4. A method, according to claim 1, and further comprising: introducing HEG into a carrier to form an assembly; and wherein, the step of introducing a volume of HEG into the shaft comprises: placing the carrier into the shaft.
 5. A method, according to claim 4, and further comprising: combining multiple layers of fabric to form the carrier.
 6. A method, according to claim 5, and wherein: the fabric comprises glass fibers.
 7. A method, according to claim 4, and wherein: the assembly comprises fifty percent HEG.
 8. A method, according to claim 1, and further comprising: obtaining stress data between the HEG and the concrete, and deflection data defining movements of the pile; and determining a pile load capacity. 